C and C++ Strings

C and C++ Strings An Array Type for Strings • C-strings can be used to represent strings of characters – C-strings are stored as arrays of characters – C-strings use the null character '\0' to end a string • The Null character is a single character – To declare a C-string variable, declare an array of characters: char s[11]; C-string Details • Declaring a C-string as char s[10] creates space for only nine characters – The null character terminator requires one space • A C-string variable does not need a size variable – The null character immediately follows the last character of the string • Example: s[0] s[1] H i s[2] s[3] M s[4] o s[5] s[6] s[7] m ! \0 s[8] s[9] ? ? Initializing a C-string • To initialize a C-string during declaration: char my_message[20] = "Hi there."; – The null character '\0' is added for you Is called a string constant or string literal • Another alternative: char short_string[ ] = "abc"; but not this: char short_string[ ] = {'a', 'b', 'c'}; – because null character '\0' will NOT be added Length of C Strings To calculate the length of a string use the strlen() function – returns the number of characters between the start of the string to the terminating character: char name[20] = "Jim"; strlen(name)  3 This should not be confused with the size of the array that holds the string. sizeof(name)  20; char emptystr[10] = ""; // same as: char emptystr[10] = {'\0'}; // or = {0}; strlen(emptystr)  0 sizeof(emptystr)  10; What about char empty[50]; strlen (empty)  ??? Assignment With C-strings • This statement is illegal: char my_name[20], your_name[20]; my_name = "Thao";  ERROR! – This is an assignment statement, not an initialization – The assignment operator does not work with C-strings (just like it doesn’t work for arrays) my_name = your_name;  ERROR! • Assignment of C-strings • To assign a value to a C-string variable is to use strcpy, defined in the cstring library #include char my_name[20]; strcpy (my_name, "Thao"); Places " Thao" followed by the null character in string my_name. A Problem With strcpy • strcpy can create problems if not used carefully – strcpy does not check the declared length of the first argument – It is possible for strcpy to write characters beyond the declared size of the array: char my_name[10]; strcpy (my_name, "Thao Thi Nguyen");  this will overwrite memory! A Solution for strcpy • A safer version of strcpy is named strncpy – strncpy uses a third argument representing the maximum number of characters to copy – Example: char another_string[10]; strncpy(another_string, a_string_variable, 9); another_string[9] = ‘\0’; This code copies up to 9 characters into another_string, leaving one space for '\0' Comparing C-strings • The = = operator does not work with C-strings – The predefined function strcmp is used to compare C-string variables – Example: #include … char name1[20], name2[20]; if (strcmp(name1, name2)) cout [...]...strcmp's logic strcmp(s1, s2) == 0 if strings are the same strcmp(s1, s2) != 0 if strings are different strcmp(s1, s2) < 0 if s1 precedes (comes before) s2 strcmp(s1, s2) > 0 if s2 precedes s1 strcmp examples cout 0 cout 1 cout -1 cout -1 ... for Strings • C- strings can be used to represent strings of characters – C- strings are stored as arrays of characters – C- strings use the null character '' to end a string • The Null character... a single character – To declare a C- string variable, declare an array of characters: char s[11]; C- string Details • Declaring a C- string as char s[10] creates space for only nine characters –... ); replace( ); Converting C+ + string to C- string Converting C- string to C+ + string • Happens automatically in most cases string s = "abc"; • Can force using string("abc") string s = "abc" + "def";